This invention relates to digital telecommunications systems and in particular to an arrangement and method for transmitting a synchronous transfer mode (ATM) traffic.
A recent development in telecommunications technology has been the introduction of the a synchronous transfer mode (ATM) transmission technique. The a synchronous transfer mode (ATM) technology is a flexible form of transmission which allows various types of service traffic, e.g. voice, video or data, to be multiplexed together on to a common means of transmission, the traffic being carried in cells each having a header indicating its destination. The service traffic is adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of the ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL). The technique allows large volumes of traffic to be handled reliably and efficiently.
There is now a need to make provision for low bit rate multiple users who wish to share an ATM connection. This has introduced the problem of the assembly delay that is encountered in assembling the user traffic for despatch.
An object of the invention is to minimize or to overcome this disadvantage.
According to the invention there is provided a method of transmitting traffic from a plurality of low bit rate users over an ATM connection via a dynamic structured data transfer (DSDT) process, the method including multiplexing the user traffic together on a trunk group or frame by frame basis.
According to another aspect of the invention there is provided a method of transmitting traffic from a plurality of low bit rate users over an ATM connection via a dynamic structured data transfer (DSDT) process, the method including changing the trunk group structure size dynamically in order to accommodate for fewer or greater low bit-rate users on the connection.
According to a further aspect of the invention there is provided apparatus for transmitting traffic from a plurality of low bit rate users over an ATM connection via a dynamic structured data transfer (DSDT) process, the apparatus including means for changing the trunk group structure size dynamically in order to accommodate for fewer or greater low bit-rate users on the connection.
Low bit-rate synchronous services can be defined as any low bit-rate (64 kb/s or lower) service that generates user information on a fixed periodic basis. For example, for 64 kb/s, 32 kb/s and 16 kb/s an octet of information is generated every 125 ms, 250 ms, and 500 ms respectively. By multiplexing on a trunk group basis using DSDT, a considerable increase in bandwidth utilisation for a given bounded cell assembly delay can be achieved.
SSCS/DSDT enables multiple low bit rate users to be multiplexed together on a trunk group basis, i.e. one minicell per frame rather than on a user by user basis, to achieve a high bandwidth efficiency for a given bounded cell assembly delay. Irrespective of trunk group size, the SSCS/DSDT mechanism can be tuned to generate SDUs of length approximately equal to the ATM payload sizexe2x80x94this yields the optimum balance between high bandwidth efficiency and eliminating potential error extension. The size of the trunk group is deterministic and thus at all times the receive station will have implicit knowledge of the length of the mini-cells that it receives.
However, it is possible to change the size of the trunk group dynamically during the lifetime of the connection to accommodate changes in the community of low bit-rate users. The structure size change is performed in a controlled manner through ANP (access node processor) negotiation and so again, the receive station has full knowledge of the new structure before the in-band change is made. DSDT can be implemented into all three of the proposed AAL-CU alternatives at present under study, and it is possible to multiplex DSDT services into the same ATM connection containing other types of services.